1. Field of the Invention
The field of the invention is that of zeolite adsorbents for the purification of gas flows contaminated by carbon dioxide, in particular for the purification of air before N2/O2 separation stages.
2. Background of the Invention
The production of pure gases, in particular N2 and O2, from atmospheric air is an industrial operation carried out on a large scale and can make use either of cryogenic processes or of adsorption processes based on the principle of pressure swing adsorption (PSA), that of temperature swing adsorption (TSA) or a combination of the two (PTSA). Furthermore, many gases resulting from industrial processes comprise significant amounts of carbon dioxide, which is often advisable to remove.
The production of N2 or O2 from air requires a purification prior to the separation stage proper. This is because, in carrying out cryogenic processes, water or carbon dioxide present in the feed air can result In blockages of the equipment due to the fact that these operations are carried out at temperatures far below the freezing points of these impurities. In the adsorption processes, water and carbon dioxide are more strongly adsorbed than nitrogen and result, in the long run, in poisoning of the adsorbent, the consequence of which is a decrease in the expected life time.
In these processes, a zeolite of faujasite type (13X, the Si/Al ratio of which is greater than 1.2) is very generally employed to Provide for the removal of the carbon dioxide, the trapping of the water generally being carried out on an alumina bed place upstream of the bed of zeolite adsorbent. The regeneration of the adsorbent is of PTSA type, that is to say that a slight rise in temperature to approximately 150xc2x0 C. is Combined with a reduction in pressure. During the stage, a fraction of purified gas produced which comprises N2, O2 and approximately 1% by volume of argon, is conveyed to the beds of adsorbents for the purpose of regenerating them by desorbing CO2 and H2O.
It is has been known for a long time that zeolite X is a better adsorbent for carbon dioxide than silica gel or active charcoal (U.S. Pat. No. 2,882,244). This patent also teaches that the selectivity with respect to various adsorbents varies with the temperature and the pressure.
U.S. Pat. No. 3,885,927 teaches that the adsorption of CO2 can be carried out on a zeolite X exchanged to more than 90% with barium: under these conditions, the CO2 content of the gas to be purified does not exceed 1000 ppm and the temperature can be between xe2x88x9240xc2x0 C. and 50xc2x0 C.
EP 294 588 teaches that a zeolite X exchanged with strontium, preferably to 70%, can also be used to carry out this purification.
The influence on CO2 adsorption of the number of exchangeable cations on the zeolite has been studied by Barrer et al. in xe2x80x9cMolecular Sievesxe2x80x9d (Soc. Chem. Ind., London, 1968), p. 233, and by Coughlan et al. in xe2x80x9cJ. C. S. Faradayxe2x80x9d, 1, 1975, 71, 1809. These studies show that the adsorption capacity of the zeolite for CO2 increases as the Si/Al ratio decreases, up to a limit of 1.2, the lower range not having been explored.
Zeollte X, the Si/Al ratio of which is close to 1.25 and which is commonly used, is very selective for CO2, this selectivity increasing as the temperature falls. At temperatures in the region of ambient temperatures, the efficiency decreases greatly as a result of the competition with nitrogen, which is present in much greater molar proportions. The N2/CO2 ratio in ambient air (with CO2xcx9c300/400 vpm) is of the order of 3000.
U.S. Pat. No. 5,531,808 discloses the teaching that CO2 can be very efficiently adsorbed by means of a zeolite of X type having an Si/Al ratio of less than 1.15 and preferably equal to or very close to 1, referred to in the continuation of the account as zeolite LSX (Low Silica X). The advantage with respect to the conventional zeolite X (Si/Al greater than 1.2) lies in the fact that it is no longer necessary to decrease the temperature at the decarbonatation stage by means of a cold unit as the efficiency of the zeolite is such that the selectivity for CO2 with respect to nitrogen remains high, even up to 50xc2x0 C.
The Applicant Company has found that the CO2 adsorption capacity of a zeolite NaLSX increases with the degree of exchange with sodium but also that the increase in efficiency begins to reach a ceiling when degrees of exchange with sodium are achieved which are of the order of 90% for relatively high CO2 partial pressures. On the other hand, the Applicant Company has shown, in WO 99/46031, that a very substantial increase in efficiency can be obtained for The decarbonatation under low CO2 partial pressures, of the order of 2 mbar, with zeolites LSX having a degree of exchange with sodium (defined as the molar ratio of the sodium ions to the aluminium atoms in the tetrahedral position, the remainder being potassium) of at least 98%.
A subject-matter of the present invention is a novel family of zeolite adsorbents comprising a mixture of 5% to 95% and preferably of 50 to 90% by weight of at least one zeolite X with an Si/Al ratio equal to 1.25 and of 95 to 5% and preferably of 50 to 10% by weight of at least one zeolite LSX with Si/Al=1 for which
either at least 80% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by sodium cations,
or at least 70% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by strontium cations, it being possible for the remainder of the exchangeable sites to be occupied by cations chosen from Groups IA, IIA and IIIA of the Periodic Table or trivalent ions from the rare earth or lanthanide series.
Mention will very particularly be made, among preferred adsorbents, of those with an overall degree of exchange with sodium of greater than 90% and advantageously greater than 98%. Mention will also be made of mixtures of zeolite adsorbents as defined above exchanged to at least 70% with strontium, the majority of the remaining cationic sites of which are occupied by sodium ions.
These novel zeolite adsorbents can be provided in the form of a powder but they can also be agglomerated in the form of beads or extrudates with 5 to 25, preferably 5 to 20, parts by weight of an inert agglomeration binder (amorohous material with a cohesive nature which has very little tendency to adsorb carbon dioxide) per 100 parts by weight of mixture of zeolite X and zeolite TSX and of binder.
The agglomerates are particularly well suited to industrial uses insofar as their handling during loading and unloading operations in an industrial unit limits the pressure drops with respect to adsorbents in the pulverulent form.
Another subject-matter of the present invention is the process for the preparation of the adsorbents as defined above.
When the adsorbents are provided in the pulverulent form, they can be obtained by simple mixing of zeolite X and zeolite LSX powders.
Synthetic zeolite X and zeolite LSX powders generally exhibit a degree of exchange with sodium of 100% and 77% respectively, the remainder of the cationic sites being essentially potassium ions.
These powders can be subjected to one or more optional cationic exchanges, either separately (i.e. prior to the intimate mixing thereof) or subsequent to the mixing stage.
These cationic exchanges consist in bringing the powders into contact with saline solutions of the cation or cations which it is desired to partially or completely insert in the zeolite structure or structures in place of the exchangeable cations already present.
Degrees of exchange are generally obtained in the conventional manner by carrying out successive exchanges with the saline solution or solutions of cations.
When the powders comprise a mixture of cations, the exchange can be carried out either via a mixed solution comprising salts of several cations or by successive exchanges of individual saline solutions, in order to insert the cations one after the other.
When the adsorbents are provided in the form of agglomerates, the stages in the production process are generally as follows:
Axe2x80x94Agglomerating and shaping the mixture of X and LSX powders with a binder,
Bxe2x80x94Drying at low temperature (of the order of 80-100xc2x0 C.) and activating at a temperature of between 300 and 700xc2x0 C., preferably between 400 and 600xc2x0 C., the product obtained in A),
Cxe2x80x94Optional zeolitization of the binder, if the binder can be converted to a zeolite,
Dxe2x80x94Washing, drying and activating, at a temperature of between 300 and 700xc2x0 C., preferably between 400 and 600xc2x0 C., the product obtained in C) or the product obtained after cationic exchange of the product resulting from B).
Mention may be made, as examples of an inert binder, of silica, alumina or clays and, as binder which can be converted to a zeolite, kaolin, metakaolin or halloysite.
The constituents of these agglomerates can be subjected to one or more cationic exchanges, followed by washing with water,
either before stage A), as indicated above for the pulverulent mixtures; in this case, the agglomerates are obtained on conclusion of stage B) or D), depending upon whether there is or is not zeolitization of the binder,
or after stage B),
or after the optional stage of zeolitization of the binder which can be converted to a zeolite on the predried products resulting from stage C) and before stage D).
If there s neither cationic exchange nor zeolitization, the adsorbent according to the invention is obtained on conclusion of stage B).
An alternative form of stage A) consists in conventionally mixing crystalline zeolite X and zeolite LSX powders with water and a binder (generally in powder form) and in then spraying this mixture over already formed zeolite agglomerates which act as agglomeration seeds. During this spraying, the agglomerates can be subjected to a continuous rotation about themselves according to a xe2x80x9csnowballxe2x80x9d type technique, for example in a reactor equipped with a rotational axis. The agglomerates thus obtained are then provided in the form of beads.
The zeolitization stage (stage C)) consists in converting the binder which can be converted to a zeolite, with which the mixture of zeolite LSX and zeolite X powders has been agglomerated beforehand, by alkaline steeping, for example according to the process disclosed in Patent Application WO 99/05063, thus making it possible to obtain agglomerates comprising little material which is inert with regard to adsorption, typically up to approximately 5% by weight of inert binder after zeolitization, which exhibits an undeniable advantage during the use of such adsorbents.
Another subject-matter of the invention is a process for the decarbonatation of a gas flow. The decarbonatation process according to the invention can be carried out by passing the gas flow to be decarbonatated over one or more adsorbent beds combined in parallel or capable of linking together the adsorption stage and the desorption stage (intended for the regeneration of the adsorbent) in a cyclical fashion; at the industrial stage, it is preferable to operate according to a process of adsorption by varying the pressure (PSA) and advantageously of adsorption by varying the pressure and the temperature (PTSA). Processes of PSA and PTSA type involve the use of pressure cycles. In a first phase, the adsorbent bed provides for the separation of the contaminant by adsorption of this constituent; in a second phase, the adsorbent is regenerated by reducing the pressure. At each new cycle, it is essential for the desorption of the contaminant to be as complete and as efficient as possible, so as to recover a regenerated state of the adsorbent which is identical or substantially identical at each new cycle.
The partial pressure of the CO2 present in the gas flow generally does not exceed 25 mbar and is preferably less than 10 mbar.
So as to continuously purify the gas flow, such as air, a number of adsorbent beds are generally positioned in parallel and are subjected alternately to a cycle of adsorption with compression and of desorption with decompression. In the PSA and PTSA processes, the treatment cycle to which each bed is subjected comprises the following stages:
a) passing the contaminated gas flow into an adsorption region comprising the adsorbent bed, the adsorbent bed providing for the separation of the contaminant or contaminants (in this instance CO2) by adsorption,
b) desorbing the adsorbed CO2 by establishing a pressure gradient and gradually reducing the pressure in the adsorption region in order to recover the CO2, via the inlet into the adsorption region
c) increasing the pressure in the adsorption region by introducing a pure gas stream via the outlet of the adsorption region.
Thus, each bed s subjected to a treatment cycle comprising a phase of producing pure gas, a second phase of decompression and a third phase of recompression.
If the only contaminant to be removed from the gas flow is CO2, only one adsorbent bed, composed essentially of agglomerates as defined above, is placed in the adsorption region.
If there are several contaminants to be removed, the adsorption region can then comprise several adsorbent beds capable of adsorbing the undesired impurities or contaminants. Thus, in order to remove the carbon dioxide and water present in air, a drying agent for adsorbing water, such as alumina or a silica gel, will be combined with the zeolite adsorbent of the present invention.
So as to optimize the PSA and PTSA processes, the phases of decompression and of compression of the various adsorbent beds are synchronized: it proves to be particularly advantageous to introduce stages for the equalization of the pressures between two adsorbent beds, one being in the decompression phase and the other in the recompression phase.
During the implementation of the process according to the invention, the adsorption pressures are generally between 0.2 and 20 bar and preferably between 1 and 10 bar, whereas the desorption pressures are generally between 0.02 and 5 bar and preferably between 0.1 and 2 bar.
As for the decarbonatation processes of the state of the art, the temperatures n the adsorption region are generally between 20 and 80xc2x0 C. and advantageously between 30 and 60xc2x0 C.; in the decarbonatation processes of the stare of the art, the regeneration temperatures which are necessary in order to obtain sufficient regeneration of the adsorbent are typically of the order of 130 to 170xc2x0 C., which makes it necessary to heat the adsorbent and increases the cost of the industrial plant.
With respect to the state of the art, the present invention offers a substantial additional advantage as regards the regeneration of the zeolite adsorbents agglomerated with a zeolitized binder according to the invention, insofar as, in order to obtain the same performance from the adsorbent after it has been regenerated, the regeneration temperatures to be employed are between 100 and 120xc2x0 C. and are thus much lower than those used to date.